Feb. 11 Meitner and Frisch publish a theoretical
interpretation of the Hahn-Strassmann results as nuclear
fission.

June-July Heisenberg visits the United States.

Aug. 2 Einstein signs letter to President Roosevelt
alerting him to the possibility of a bomb and urging
government-sponsored research.

Sept. 1 Bohr and John Wheeler publish a comprehensive
theory of nuclear fission.

Sept. 3 War breaks out in Europe.

Sept. 16 The German Army Weapons Bureau assembles
scientists to begin fission research.

Oct. 5 The Weapons Bureau takes control of the
Kaiser-Wilhelm Institute for Physics in Berlin-Dahlem.

Dec. 6 Heisenberg submits to the Weapons Bureau the first
part of a two-part comprehensive report on the prospects and
methods for exploiting nuclear fission.

1940

Feb. 29 Heisenberg submits the second part of his report
to the Weapons Bureau.

May 3 German troops occupy Norway, seizing the world's
only heavy-water production plant at Vemork.

May 19 Frisch and Rudolf Peierls submit a memorandum to
the British government estimating the critical mass of 235 U
needed for an atomic bomb and urging a bomb research
project.

June 15 Using the Berkeley cyclotron, Philip Abelson and
Edwin McMillan demonstrate that neutrons captured by 238U
lead to the creation of elements 93 and 94, neptunium and
plutonium.

July 17 C. F. von Weizsäcker suggests to the Weapons
Bureau that neptunium bred in a reactor can be used as the
explosive material in a fission bomb.

1941

Jan. 20 Walther Bothe and Peter Jensen report results on
neutron absorption in graphite indicating, mistakenly, that
graphite cannot be used as a moderator.

Mar. 28 American physicists confirm that plutonium is
fissionable, thus usable for a bomb.

June 22 Germany invades the Soviet Union. August Fritz
Houtermans reports to German authorities the possibility of
using plutonium in a bomb.

Dec. 5 In the wake of total mobilization Erich Schumann,
head of research in the Army Weapons Bureau, orders a review
of all research projects.

Dec. 6 The Manhattan Project to build the bomb is
launched.

Dec. 7 Japan attacks Pearl Harbor; America enters the
war.

1942

February The Army Weapons Bureau decides to withdraw
almost entirely from fission research and relinquishes the
Kaiser-Wilhelm Institute for Physics.

Feb. 26 Heisenberg, Hahn, and other scientists deliver a
lecture series on nuclear research to the Reich Education
Ministry in Berlin, gaining ministry backing for the project
under the Reich Research Council.

April The first neutron multiplication is obtained in a
Leipzig test reactor.

[June 4] Heisenberg reports on fission research to Albert
Speer, Germany's Minister for Armaments and War Production,
and other senior officials.

June 9 Hitler issues a decree, placing the Reich Research
Council under Goring and Speer.

July 1 Heisenberg becomes acting head of the Kaiser-
Wilhelm Institute for Physics, Germany's main reactor
research laboratory, and lays plans for the construction of a
working reactor containing heavy water and uranium metal
plates.

July Kurt Diebner, supported by the Weapons Bureau,
begins reactor construction using the alternative design of
metal cubes suspended in heavy water, achieving positive
neutron multiplication over the following year.

Dec. 2 Enrico Fermi and collaborators in Chicago achieve
the first self-sustained chain reaction in a pile consisting
of uranium spheres embedded in graphite bricks.

1943

January Planning begins for construction of reactors at
Hanford, Washington to breed plutonium for a bomb.

May 6 Heisenberg, Hahn, and other scientists deliver
lectures on fission research before Göring's German Academy
of Aerodynamical Research.

Autumn Berlin research institutes begin moving to
southern Germany for safety against Allied bombing raids. The
Kaiser- Wilhelm Institute for Physics is split between Berlin
and the neighboring southern towns of Hechingen and
Haigerloch.

1944

[Jan. 1] Walther Gerlach is appointed "plenipotentiary"
of all fission research sponsored by the Reich Research
Council.

June 6 D-Day invasion of Europe.

August The Alsos Mission, an American science
intelligence unit, arrives Europe.

November The Alsos Mission determines that no German atom
bomb exists.

1945

January Gerlach orders the remainder of the Heisenberg
and Diebner teams to move south.

March The Heisenberg team in Haigerloch begins war time
Germany's last attempt to achieve a critical reactor.